Bischofite

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Bischofite
Mineralogical name	Bischofite
Chemical name	Magnesium chloride hexahydrate
Trivial name
Chemical formula	MgCl2•6H2O
Other forms
Crystal system	monoclinic
Crystal structure
Deliquescence humidity 20°C	33.1%
Solubility (g/l) at 20°C	5.75 mol/kg
Density (g/cm³)	1.57 g/cm3
Molar volume	129.6 cm3/mol
Molar weight	203.30 g/mol
Transparency	transparent to translucent
Cleavage	none
Crystal habit
Twinning
Phase transition
Chemical behavior
Comments	decomposes at 116-118°C hygroscopic and deliquescent
Crystal Optics
Refractive Indices	nx =1.495 ny = 1.509 nz = 1.528
Birefringence	Δ = 0.003
Optical Orientation	positive
Pleochroism
Dispersion	79°
Used Literature
[Broul.etal:1981]Author: Broul M., Nyvlt J.; Soehnel O. Editor: Elsevier Publisher: Elsevier Title: Solubility in organic two component systems Year: 1981 [Dana:1951]Author: Dana J.D. Edition: 7 Editor: Dana E.S. Publisher: Wiley & Sons Title: Dana's System of Mineralogy Volume: 2 Year: 1951 [Steiger.etal:2011a]Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy Journal: Geochimica et Cosmochimica Act Number: 12 Pages: 3600-3626 Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars Volume: 75 Year: 2011

Solubility

Bischofite has got a high solubility in water (5.75 mol/kg at 20 °C). The solubility of bischofite and other phases of the system MgCl₂-H₂O is shown in Fig. 1.

Figure 1: Solubility of magnesium chloride in water. The molality m [n(MgCl₂)•kg(H₂O)^-1] is plotted versus the temperature.

Hygroscopicity

Figure 2 includes the phase diagram of the system MgCl₂-H₂O. Next to the hexahydrate bischofite there are two more stable phases within the regarded temperature range. The octa- and the dodecahydrate are relevant at lower temperatures.

Figure 2: Phase diagram of the system MgCl₂-H₂O.The water activity a_w is plotted versus the temperature.

Table 1: Deliquescence humidities of bischofite at different temperatures [Steiger.etal:2014]Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja
Booktitle: Stone in Architecture
Chapter: 4
Editor: Siegesmund S. and Snethlage R.
Pages: 223-316
Publisher: Springer Verlag Berlin Heidelberg
Title: Weathering and Deterioration
Year: 2014

0°C	10°C	20°C	30°C	40°C	50°C
34.1%r.h.	33.7%r.h.	33.1%r.h.	32.4%r.h.	31.5%r.h.	30.5%r.h.

Literature

[Broul.etal:1981]	Elsevier (eds.) Broul M., Nyvlt J.; Soehnel O. (1981): Solubility in organic two component systems, %edition%, Elsevier, %address%, %pages%, %url%, %doi%
[Dana:1951]	Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & Sons, %address%, %pages%, %url%, %doi%
[Steiger.etal:2011a]	Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy (2011): Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars. Geochimica et Cosmochimica Act, 75 (12), 3600-3626, %url%, 10.1016/j.gca.2011.03.038,
[Steiger.etal:2014]	Steiger, Michael; Charola A. Elena; Sterflinger, Katja (2014): Weathering and Deterioration. In: Siegesmund S.; Snethlage R. (eds.): Stone in Architecture, Springer Verlag Berlin Heidelberg, 223-316, %url%, 10.1007/978-3-642-45155-3_4